Air Pod filters will work, you just have to be smart about it!

90% of the reason Air Pods or individual filters on each carburator don't work is the non-laminar air flow they allow or create through the carburator body/venturi. Air pods sit open to the environment, dynamic air, flowing past the irregular shape of the bike and the rider causing turbulence and vortex's. As the down stroke of the piston causes air to be drawn into the carb through the air filter. In the case of the stock air box the air is sucked in through a common air filter and then down stacks into the individual carbs. The common filter and collection box calms the air and eliminates much of the turbulent outside environment. Any remaining disruption is smoothed out by the velocity stack as it approaches the carb throat, as depicted in this first primitive drawing.



When you remove the OEM air box and place Air Pods on each carb, the draw of air is pretty much from any where on the pod. And with the pod sitting in dynamic environment the pod is very vulnerable to the turbulences and vortex currents, in other words the air being drawn in is not very calm. And because of it not being calm or smooth the air flow through the carb throat is not consistant. You can see again in this drawing what I mean...



Now this is very important, why? The carb is a metering device providing a control air/fuel mixture to each cylinder. The amount of fuel is controlled by a jet which controls by it's position a determined amount of fuel to flow, inactual fact to be sucked up through the jet by the flow of laminar air flowing over the jet. If the flow over the jet is turbulent or inconsistant the amount of fuel is affected drastically. The stack in the OEM air box and the air box itself allows for a smooth flow of air over the jet. And in the Air Pod diagram you can see that the flow is all over the map coming from all angles creating a turbulent situation.

Don't give up because there are ways to be successful using the Air pods. If you create a simple stack inside the individual Air Pod you can reduce the disruption and produce a quieter more laminar flow. Effectively you are still using the same surface area of the filter, you are just not letting the air to enter the throat of the carb at right angles at the edge of the carb horn. In fact you are forcing the air to have to flow into the carb throat at the open end of your simple stack at a point further away from the jet allowing for the flow to smooth out and become more laminar before it passes over the jet openning. You can see this in this last diagram...



Now this will improve your chances at finally tuning your bike to function the way it did with the stock OEM air box. I chose to use an external shroud over the four carbs which essentially achieved the same affect as four individual internal stacks. Yes there will be the nay sayers that will argue the theory or the validity of what I say, but I have tuned many a bike from single to duals to quad carbed bikes and never had to give up on Pod filters.

Great article. Thanks!

I guess this is even more important using cv carbs.

I see all the hate for pods here and I scratch my head. Pods and CV's where what almost everybody did starting back in the '70's and through the '90's until the factories started actually paying attention to airbox design. Million dollar businesses like Dynojet and FactoryPro sprang out of this era. Before them you spent the time learning and tuning until you got your bike to run well, and it was not impossible, and you very often gained some real power. This applied to CV or conventional slide carbs. Consider the fact that from about 1980 on, most all bikes came with CV types on them, the KZ1000J and ELR's, the GS series, and the CB's. Get a Kerker, a set of K&N's, jet it up and hang on. That's what we did. Dynojet just made it easier for the non-tuning inclined guys to do that too. The best mod I ever did to my hot rodded '77 KZ1000 was putting a set of early GSXR-750 CV flatslide's and K&N's on it. It felt like I'd put fuel injection on it. This was after trying stock carbs, rejetted stockers and filters, 29mm smoothbores and 33mm smoothbores.
Pods and carbs can get along, it just takes work. Tips like this thread's OP are a big help too.

Absolutely correct Shoeman, I fought hard tuning CV carbs with Pods on a dyno then felt sick when I dynamically test the setting on the road. Then I spent some time conversing with an aeronautical engineer attempting to understand the reason why the settings failed miserably under realtime use. My experience on the road went like this, first ten miles the bike pulled hard through all gears. Occasionally I felt stumbles at times through the rev range, but couldn't pinpoint a spot where I could predict a recurrance. Further into the ride holding the revs in several gears I saw a pattern, air flow past the carbs was having an affect on performance. I felt the bike fall flat on it's face when I road from a wooded area out into an open field area where crosswinds were serious. I also experienced the same serious drop in power, almost like I lost spark, when I took my hand off the lefthand grip to wave at on coming fellow riders, or if I moved my knees out away from the tank sides.

It was this repeatable condition that gave my engineer friend and myself enough information to start developing means to correct the problem. The simple solution was the wrap around shroud that you can see in the attached photo of the above thread. The same result could have been gained by installing individual internal stacks in each filter. Tapered filters do present a problem with the length of the internal stack, non-tapered filters allow for longer stacks which provide a greater calming of the air flowing down the carb throat.

My recommendation to fellow builders tune the carbs as best as you can on the table or if you have access on a dyno, then take the bike out for a lengthy ride doing plug chops at several throttle points, especially at points where flat spots occur. This will help determine if the flat spot is a lean or rich condition. Then experiment with either shrouds or stack tubes and see if the performance improves... it's not a perfect fix but you will be amazed at the improvement.

Dyno results please.

Otherwise it's all for nothing.

it's still one of my favorite videos ;D

https://www.youtube.com/watch?v=48Ynnsw6lJA

DohcBikes said:

Dyno results please.

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I'm not sure I've ever seen any. At least none that prove positive results.

I really, really hope it can happen. PJ does an excellent job modifying the CB360 CV carbs to handle the K&N rc-1820 pods. It's very clear results I've experienced on the road myself, but even those I have not seen dyno results.

Side by side dyno graphs of the results will prove the issue. I doubt you will find that the change of only airbox to pods will show the pods making more power than the box. If there are other changes those must be tested with the airbag also for an accurate result. I've never seen a dyno result that showed pods were better even though you may get them to run acceptably.

No where in any of my statements have I suggested that Air Pods will improve performance, I am just saying that with these minimal modifications incombination with Pods you can get an older style carbed bike to run without flat spots or detrimental conditions. And to give yourself a fair chance of being successful stay away from EGay Air Pods and use K&N as their flanges don't have an inner reduced rubber ring that blocks off vital air jets or passage ways.
I get free use of a resistance drum and I use my own air/fuel sensor which I use to read exhaust gases to determine lean or rich conditions. The set-up I have access to does not have charting, data logging for torque or HP determination, wish it did but no such luck.

If you dyno it, you will find that you lost power through most of the rpm range. In fact, in all the dyno runs I've done which is hundreds, I've seen plenty of times where restricting the air compared to stock gained the most power. Noise does not equal horsepower.

+1 there in most cases altering OEM set ups result in less performance across the rpm band. But alot want the look of custom exhausts and custom open or pod style filters even if there is factual information to show the loses. Some will make these modifications and ride their bikes tolerating terrible performance and down right damaging conditions for the visual esthetics.

In my case the combination of the pods and custom pipe allowed for greater upper end performance and that was judged by seat of the pants and speedometer readings, which were GPS validated.

In the case of my CB450 I used internal stacks to remove the 3/4 throttle flat spots I found while trying for top speed runs. Going back to a stock airbox was not an option in that build...

I'll stand by my statement that back in the day swapping the airbox for individual filters could often provide a useful power gain. Old airbox designs were for the most part designed with only a secure position for the filter in mind, and ease of access to the filter as well. The typical liter bike of the late '70's fed it's 1000+/- cc engine through a hole up under the seat about 2" in diameter.
Here's a shot of a page out of Bell's 4 stroke tuning book that shows, in his usual careful manner, a series of dyno tests that do in fact show some good hp gains from removing the airbox and using individual filters.
I'm not arguing your dyno results, DOHC, but I'd then wonder that if restricting airflow more than stock in some cases yields an increase, might that indicate a lean condition that was partially corrected by the reduction in airflow? Would rejetting those carbs and using the stock airbox possibly yield a gain?
One could argue that the power curve suffers in some way often by mods such as these, and that is true. The results in the picture illustrate that. Gain some at 8K, lose some at 4k. If that is acceptable really depends on the characteristics of the curve you are after. Modern high performance bikes take full advantage of airbox design, but that's tough to implement on the old dogs most of us are fooling around with here, some part of the frame is always in the way it seems.
It's been a long week at work and it's Friday night. Time for a beer!

Shoeman said:

I'll stand by my statement that back in the day swapping the airbox for individual filters could often provide a useful power gain. Old airbox designs were for the most part designed with only a secure position for the filter in mind, and ease of access to the filter as well. The typical liter bike of the late '70's fed it's 1000+/- cc engine through a hole up under the seat about 2" in diameter.
Here's a shot of a page out of Bell's 4 stroke tuning book that shows, in his usual careful manner, a series of dyno tests that do in fact show some good hp gains from removing the airbox and using individual filters.
I'm not arguing your dyno results, DOHC, but I'd then wonder that if restricting airflow more than stock in some cases yields an increase, might that indicate a lean condition that was partially corrected by the reduction in airflow? Would rejetting those carbs and using the stock airbox possibly yield a gain?
One could argue that the power curve suffers in some way often by mods such as these, and that is true. The results in the picture illustrate that. Gain some at 8K, lose some at 4k. If that is acceptable really depends on the characteristics of the curve you are after. Modern high performance bikes take full advantage of airbox design, but that's tough to implement on the old dogs most of us are fooling around with here, some part of the frame is always in the way it seems.
It's been a long week at work and it's Friday night. Time for a beer!

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yes, many bikes are jetted lean from the factory but dont forget about volumetric efficiency. there is no need to overfuel a bike that wont use it. On many test bikes you will not gain power by opening the intake and jetting up because the engine simply is not capable of using that much fuel and air.

one does not ARGUE that it will usually negatively affect the hp curve in some way, one KNOWS it usually will. And by usually I mean almost always.

Nice data.

I like my power in the midrange.

Very nice explanation. Follows my experience exactly, Thanks. I would add that as bikes have become newer (from the '70's) it seems like the OEM air boxes become more performance oriented. The older ones seem to have been more about intake howl and filter maintenance functionality so scrapping them had more potential benefit. Never tried the shroud concept myself but had considered it at one time. Cool to see it implemented and found useful!

Personally, I'm more interested in torque. Where's the torque?

jpmobius said:

Very nice explanation. Follows my experience exactly, Thanks. I would add that as bikes have become newer (from the '70's) it seems like the OEM air boxes become more performance oriented. The older ones seem to have been more about intake howl and filter maintenance functionality so scrapping them had more potential benefit. Never tried the shroud concept myself but had considered it at one time. Cool to see it implemented and found useful!

Click to expand...

I've been telling people to do "the shroud thing" for as long as I can remember. No offense, really, but this is not a breakthrough

What about the RS1000 manual from Honda? Anyone have dyno results from those modifications?

Velocity stacks should give a similar effect to what cqyqte talks about but I'm really curious to know what kind of performance boost they were getting, it is a racing manual after all..

Cheers

Or fab up some sort of extension between the carbs inlet side and the individual filter. That added length gives the air column time to straighten out, reducing the turbulence as a result. You can also play around with varying the length of the extension as a means of fine tuning the inlet tract length and it's effect on cylinder filling, but that's a study for another day, and often in reality a frame tube or something will get in the way so there is little room to allow experiments like this anyways. But the theory can be a fine way to occupy your time and give the brain a workout.

deviant said:

Personally, I'm more interested in torque. Where's the torque?

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One way of increasing torque is cam timing(reducing overlap?), and tuning intake and exhaust resonance, finding that optimum volumetric efficiency dohc is talking about.
Now im a humble novice in this field but none of my mods are pulled out of my ass, its all been taught by knowledgable tuners and it all does work.